Absorption
Prepared by Dr.Nagwa El-Mansy
Cairo University
Faculty of Engineering
Chemical Engineering Department
Forth year
References:1-Coluson and Richerdson, Chemical Engineering vol ,
vol II , vol III.
2- Geancoplis, Principles of Unit Operation.
3- Mc-Cabe and Smith, Unit operations for Chemical
Engineering.
4- Traybal, Mass Transfer Operations.
5- Sherwood, Mass Transfer.
6-Perry’s , Chemical Engineering.
7- “Separation Process Principles”, 2nd ed, Seader et’al .
8- Site on Google search, Separation Processes.
Absorption
Absorption:It is a gas-liquid mass transfer operation in which liquid
solvent is contacted with gas mixture for differential
dissolution of one or more components of gas and
provide a solution of them in liquid.
Uses of absorption:1- Purification of gases (H₂S from HC’s).
2-Separation of gases (separation of dry gas [C₁,C₂]
from LNG [C₃,C₄].
3- Production of useful liquid product:HCL (g) + H₂O (liq) → HCL (liq)
2NO₂(g) + H₂O (liq) → HNO₃ + HNO2
SO₃ (g) + H₂O (liq) → H₂SO₄
Applications of absorption:
1- Hydrogen sulfide(H2S) is removed from hydrocarbon
gases by washing with alkaline solution (Amines).
2- Washing ethanol vapors from carbon dioxide from
molasses fermentor tanks with water to remove
ethanol.
3- Acetone can be recovered from acetone-air mixture
by passing the gas stream into water in which acetone
is dissolved while air is passed out.
4-Carbon dioxide present in air is absorbed by sodium
hydroxide (NaOH solution) in which chemical
absorption takes place.
5- Nitrogen oxides are absorbed in water to give nitric
acid.
6- Removal of ammonia coming from coke ovens by
water
Choice Of Solvent For Gas Absorption
The factors to be considered are
High absorption power
Which means that gas solubility should be high in the
solvent, which results in increasing the rate of
absorption and decreasing the quantity of solvent
required.
Highly Selective
The selectivity of solvent must be high in which solvent
dissolve one and leave the others.
Easy to recover
Which means easily to be regenerated.
Low volatility
The solvent should have a low vapor pressure to
reduce loss of solvent in the gas leaving the
absorption column.
Small viscosity
Low viscosity is preferred for reasons of rapid
absorption rates, improving flooding characteristics
in packed column, low pressure drops on pumping
and good heat transfer characteristics.
Cost
The solvent should be inexpensive, so that losses
are not costly, and should be readily available.
Other properties
Non-toxic, Non-flammable, Non-corrosive,
Chemically stable, low freezing point
Absorption Equipments
(A) Plate Towers:1-Multistage contact.
2-High separation , high capacity.
3-Relatively large diameter.
4-Cooling is done by
providing the plate
with cooling coils.
5- High pressure drop.
6- Easy to be clean.
(B) Packed Columns
1-Differential contact.
2-Used for highly
corrosive materials.
3- Small diameters <70-80 cm
4-Not easy to clean.
5-Packing materials are
made from(ceramics ,
bricks, wood, gravels,
stones , steel ,……)
6-To increase surface area of contact between the two phases in
packed columns, make more than one section which increase
the performance of
the tower.
7- cooling is done by dividing
the column
To many sections
out side the column
(as seen in the opposite
Figure).
(C) Spray Column:1- Continuous contact.
2- Low pressure drop.
3- Low efficiency.
4- Low cost(empty).
5- Gas phase controlling.
6- Considered as one stage.
(D) Wetted wall Column:Single tube wetted wall column used
in labs for measuring mass transfer
coefficient.
(E) Tubular Reactor:1- used for highly
exothermic reactions.
2-for highly heats of
reactions.
3- proper for heat
transfer control.
4- low mass transfer
due to small surface
area of contact.
(F) U-Tube Absorber:1- Specially for highly corrosive
materials(HCL)
2- Small surface area of contact
between two phases.
4-Simple in construction.
5-Use any material of
construction( ceramic, cast iron,
silicon,……)to over come
corrosion problems.
6-Very difficult in casting and
welding.
(G)Centrifugal type of
absorption:1- Single stage absorber.
2-Co-current contact
3-Used for highly viscous
liquids.
4-Used for foamy liquids.
5-Liquids are sprayed by
centrifugal force.
6-Provied good contact
between two phases.
7-Operating and initial cost
are very high.
Equilibrium Relations:Mass transfer between G/L depends
highly on the equilibrium between
G/L. Different gases and liquids yield
separate solubility curves , which
must be determined experimentally
for each system. If the equilibrium
pressure of a gas at a given liquid
concentration is high, as case (A) in
the opposite figure, the gas is said to
be relatively insoluble in liquid ,
while if its low, as for curve (B) , the
solubility is said to be high.
Effect of temperature on the
equilibrium curve:The solubility of any gas is
influenced by the temperature. If
the temperature of the system at
equilibrium is raised , the solubility
of a gas decreases . As shown in
the opposite figure as temperature
increases for the same solute (gas)
the solubility decreases from (1060)oC and the absorption power
decreases .
Absorption process is usually
accompanied by evolution of heat.
So It is necessary to fit coolers to
the absorber to keep its
temperature sufficiently low.
Effect of temperature on the equilibrium curve
Types of Equilibrium Relations :For dilute concentrations of many
gases the equilibrium relationship is
given by Henry’s law which relates
the partial pressure developed by a
dissolved solute(A) in a liquid
solvent (S) by the following
equation:PA = H xA
Where:H is Henry’s constant expressed as
kPa / mole fraction solute in liquid,
PA is the partial pressure of solute in
kPa,
xA is the mole fraction of the gas in
liquid phase
Henry’s law holds very well when the
partial pressure of the solute is less
than atmospheric. Above
atmospheric pressure , H may be
independent of the partial pressure.
The variation of H with temperature is
strongly nonlinear function.
For ideal systems Raoult’s law is valid:PA = PoA xA
Where
PA , is the partial pressure of solute .
PoA , is the vapor pressure of solute.
xA , is the mole fraction of the solute in
the liquid phase.
PA = H A x A
PA
HA x A
=
PT
PT
(by dividing each tearm by PT )
y A = m* x A (where x A andy A are mole fractions)
Conversion from mole fraction to mole ratio:mole fraction of A in gas phase y A =
nA
nA + nB
mole fraction of A in liquid phase x A =
nA
n A + nS
nA
mole ratio of A in gas phase y A =
nB
,
mole ratio of A in liquid phase x A =
nA
nS
,
PA = H A x A
nA
m* n A
=
nA + nB
n A + nS
m*n A /n S
n A /n B
=
n A /n B +n B /n B
n A /n S +n S /n S
YA
m* X A
=
YA +1
X A +1
YA
m*X A
=
YA +1-YA
X A +1- m*X A
m*X A
 YA 
X A +1- m*X A
or
YA 
m*X A
(an equilibrium relation at certain temperature and pressure)
X A (1- m*)+1
Factors affecting absorption process:A- Choice of solvent flow rate :Usually given:1- gas flow rate(Gin).
2- feed composition(yin).
3-solvent composition( xin ).
4-degree of separation= sharpness of
separation.
recovery= (Yin – Yout )/ Yin
in which we can calculate the outlet gas
composition.
Yout = Yin (1 – recovery)
Here we want to calculate proper
solvent rate(Lin)
By using mole or mass ratios we must remove the amounts of
solute from gas and liquid flow rates as flows:Ginert = G’ = G in ( 1 – yin) . Where yin is feed mole fraction
L inert = L’ = L in ( 1 – x in). Where xin is solvent mole fraction
By making material balance on the absorber:
G’ Yin + L’ Xin = G’ Yout + L’ Xout
G’ ( Yin – Yout ) = L’ ( Xout – Xin)
L’ / G’ = (Yin – Yout) / (Xout – Xin)
[ Operating line equation]
Operating line is a line between two points (Xin,Yout) and
(Xout, Yin) and has a slope - L’/G’
As the amount of solvent decreases (L’) the slope of the operating
line decreases and goes down and the number of stages increases.
The effect liquid amount on the number of stages:As the amount of liquid solvent decreases the driving force
decreases and the number of stages increases thus the tower
cost increases till operating line cuts or touch the equilibrium
curve at this point we reach pinch point which means no
separation.
But if we increase the amount
of liquid solvent ,the slope of
the operating line goes up
and the driving force
increases which means small
number of stages is
required( also small number
of transfer units). This
means that we must make
optimization for liquid
amount as shown in the
opposite figure.
We have to use L/G > ( L/G)Min
(L/G) Opt =[ 1.2 to 2.5 ] (L/G) Min
(B)Temperature:In general absorption process
is an exothermic process , it
improves by lowering
temperature. Thus we make
good cooling for liquid
solvent before entering the
column. Increasing
temperature results in:1- equilibrium curve goes up
and absorption power
decreases.
Notice that for the same Y the
separation increases with
decreasing temperature as
shown in the opposite
Figure.
X3 < X2 < X1
2-For same (L/G), Number of stages (or NTU) increases , means
tall column and high cost ,which is bad conditions.
3-for same(L/G) , driving force decreases and separation
becomes difficult ,which is a bad conditions
In some cases even refrigeration is
economic, this happen when
losses in solvent is high ( to
minimize losses = economic).
Some times average slight
increase in temperature is
permissible and have +ve
effect when:1- Solvent has high viscosity.
2- Case of chemical reaction, in
which rate of absorption is
affected positively by increasing
temperature.
The highest temperature (T Max)
in the absorber can be found at
the bottom of the column.
(C)Pressure:As pressure increases absorption
power increases
PA = HA xA( Henry’s law)
PA / PT = (HA / PT) xA
y A = m* xA
Increasing pressure results in:1- the equilibrium curve goes
down which improves the
absorption process.
Notice that for the same (Y) the
separation increases with
increasing pressure as shown
in the opposite figure
X3 < X2 < X1
2-For same (L/G), Number of stages (or NTU) decreases ,means
short column and low cost which is good conditions
3-Driving force increases and separation becomes more easier
which means good separation.
Physical vs chemical absorption:There are 2 types of absorption processes: physical absorption and
chemical absorption, depending on whether there is any chemical
reaction between the solute and the solvent (absorbent).
When water and hydrocarbon oils are used as absorbents, no
significant chemical reactions occur between the absorbent and
the solute, and the process is commonly referred to as physical
absorption.
When aqueous sodium hydroxide (a strong base) is used as the
absorbent to dissolve an acid gas, absorption is accompanied by
a rapid and irreversible neutralization reaction in the liquid
phase and the process is referred to as chemical absorption or
reactive absorption.
More complex examples of chemical absorption are processes
for absorbing CO2 and H2S with aqueous solution of mono ethanolamine (MEA), di -ethanolamine (DEA), diethyleneglycol (DEG) or tri-ethyleneglycol (TEG), where a
reversible chemical reaction takes place in the liquid phase.
Chemical reactions can increase:1- the rate of absorption.
2- increase the absorption capacity of the solvent.
3-increase selectivity to a certain components of the gas, and
convert a hazardous chemical to a safe compound.
Physical absorption:-
Chemical absorption:-
A solute of gas (A) is absorbed from a mixture by solvent liquid(B),
which combines with (A) according to the equation A + B→ AB.
As the gas approaches the liquid interface, it dissolves and
reacts at once with (B). The new product(AB),thus formed ,
diffuses towards the main body of the liquid.
The concentration of (B) at the interface falls; this results in
diffusion of (B) from the bulk of the liquid phase to the
interface. Since the chemical reaction is rapid,(B) is removed
very quickly, so that it is necessary for the gas (A) to diffuse
through part of the liquid film before meeting (B). There is a
zone of reaction between A and B which moves away from the
gas-liquid interface. The final position of this reaction zone will
be such that the rate of diffusion of (A) from the gas-liquid
interface is equal to the rate of diffusion of (B) from the main
body of the liquid.
Eight distinct kinetic regimes are observed. For instantaneous ,
reaction or for rapid chemical reaction, the reaction occurs only
in liquid film during the transportation of component(A). The
concentration of (A) in the bulk of the liquid is zero(rate of
reaction of A ((rA)=0), such as absorption of acid gas. These
reactions are characterized by Hatta number (Ha) :Ha = max possible conversion in liquid film/max diffusion transport through the liquid film
= (K’ CBo δ2L /DA)> 3 where:- K’ = the reaction constant.
CBo = liquid concentration
δ2L = liquid film thickness
DA = diffusivity of solute A
At the other extreme, for very slow chemical processes occurs in
the liquid bulk no reaction occurs in the film and mass transfer
is used to keep the bulk concentration of component (A) close
to the saturation value ( CA = CA*). These reactions are
characterized by Ha<< 1 such as oxidation, hydrogenation.
Notice that chemical reaction affect the equilibrium curve.
Effect of temperature on the absorption tower:Many absorbers and strippers deal with dilute gas mixtures and
liquids, in these cases it’s assumed that the operation is
isothermal. But actually absorption operations are usually
exothermic, and when large quantities of solute gas are
absorbed to form concentrated solutions, the temperature
effects cannot be ignored. If by absorption the temperature of
the liquid is raised to a considerable extent, the equilibrium
solubility of the solute will be appreciably reduced. Cooling
must be done to over come the increase in liquid temperature.
Consider the tray tower shown in the figure. If Qc is the heat
removed per unit time from the tower by any means.
Enthalpy balance :For non-adiabatic operation:G ( Yin – Yout) q = L cp ( Tout - Tin ) + Qc .
For adiabatic operation.
G (Yin –Yout) q = L cp (Tout -Tin )
Cp = specific heat for pure liquid.
q = heat of absorption (J/mole).
By studying the adiabatic operation there
will be some assumptions must be
considered:1-No heat is removed inside the tower,
Qc= zero.
2-All the amount of heat due to absorption
increase the liquid temperature only.
3-No evaporation in the liquid solvent
(no losses).
To estimate the temperatures inside the
absorber, the heat balance equation to
compute the temperature of the liquid
leaving each plate from the top to the
bottom ,is as shown in the opposite figure:Section (1):L ( X1 - Xin) q = L Cpliq (Tout 1 -Tin )
X1 – Xin = [Cp liq /q ] ( Tout 1 - Tin )
Section (2):L ( X2 – Xin ) q = L Cp liq ( Tout 2 – T in )
X2 – X in = [Cp liq / q] ( Tout 2 – T in )
After calculating X1 , X2 , X3 ,……., we must
plot new equilibrium curve differ than
the case of isothermal absorption.
Multi- component absorption:(A) Graphical method:The procedures for multicomponent absorption are identical for
binary mixtures. Instead of having a single equilibrium curve and
operating line, there are now an equilibrium curve and operating
line for each absorbed component of the gas. Gas flow rate(G)
and liquid flow rate(L) are approximately constant through the
column. The operating line is located with the point (xin,yout)
and the slope (L/G) for the key component(the component which
has more data), and because the feed composition is known we
can locate the terminal point of the operating line (xout,yin). Now
the number of stages required for specified recovery can be
determined by stepping-off stages from the other end . Exactly
the same number of stages are available for the other
components. Also the operating lines must have the same slope.
Thus we can calculate the recovery for each entering component.
Equilibrium relations may be based on the mole fraction
(B) Analytical method:Kremser equation represents an
analytical solution to a classical
separation problem of N ideal
equilibrium stages concerned with
countercurrent gas and liquid flow.
The equilibrium and operating
relations are assumed to be linear.
By using the data of the key
component and by calculating the
absorption factor (A=L/m*G) we can
calculate number of stages from the
following chart. After calculating N
we can calculate the recovery for
each other component.
Stripping (desorption):Stripping is the opposite of
absorption and involves the
removal of dissolved gases in
liquid by stripping agent.
Purpose of stripping:1- recover the dissolved solute.
2- recover the solvent.
3-to recover both solute and
solvent.
Usually absorption is followed
by stripping or desorption.
The most commonly used
stripping agent is steam.
Good stripping agent must be:1- easily condensed.
2- easily separated from the material stripped.
Equilibrium relations:(As absorption)
The following points must be
taken into consideration:1-Operating line is under the
equilibrium curve.
2- slope of the operating line
= L/G= (Y’in-Y’out / X’out-X’in )
3- As (L/G) decreases, G
increases , operating line
goes down, driving force
increases, N decreases and
NTU decreases.
4-As (L/G) increases, G
decreases, operating line
goes up, driving force
decreases, N increases and
NTU increases, till we reach
pinch point.
Special types of absorbers:(1) Absorber with reboiler
(combined absorber/stripper)
When wet gas( C2H6/ C3H8/C4 H10) is
contacted with oil solvent it dissolve
small amount of C2H6.
C2H6 can be concentrated in the gas
stream leaving the column by heating
the rich liquid oil stream to strip out
C2H6.
(2)Absorption with two
solvents
Recovery of highly volatile solvent
e.g. recovery of C5 from C3 and C4.
Solvent should have low vapor
pressure to minimize losses in it’s
amount. A second less volatile
solvent (Kerosene)can be used to
recover the evaporated portion
of the first solvent (Benzene).